A cerebral aneurysm is an expansion of an artery in the brain into the form of a lump or balloon. Aneurysms are often located behind other blood vessels and at various angles. They may be difficult to reach. Moreover, access to a cerebral aneurysm is through a very small opening.
A cerebral aneurysm clip is a surgical instrument which clips the base part of a cerebral aneurysm to temporarily or permanently isolate it from the cerebral artery. For this purpose, the clip must maintain its pressure with high reliability as long as desired without injury to the wall of the blood vessel. Such injury might be caused, for example, by a shearing action of the clip arms, which results from improper alignment; improper clipping pressure; foreign material trapped in cracks and crevices formed in the clip design; surface imperfections on the clip material which can tear tissue; or the use of unsuitable materials to manufacture the clip.
FIG. 1 illustrates a conventional cerebral aneurysm clip 10. Clip 10 has a pair of blades 12 and 14 which are positioned to face each other. A coil spring 20, generally called a "torsion" spring, is formed between the base ends 16 and 18 of blades 12 and 14. Typically, coil spring 20 has one-and-one-half (as shown in FIG. 1) or two-and-one half coils. The free ends 22 and 24 of blades 12 and 14 clip the aneurysm. Blades 12 and 14 are opened and closed using the base end 26 of coil spring 20 as a fulcrum. The elasticity of coil spring 20 provides clip 10 with its clipping force.
FIG. 2 shows how conventional cerebral aneurysm clip 10 is applied using an applicator 40. Applicator 40 has a pair of jaws 42 and 44 which envelop and engage the bases 32 and 34 of blades 12 and 14 of clip 10. (As shown in FIG. 2, conventional applicator 40 is larger than the clip 10 which it applies; therefore, the combination of clip 10 and applicator 40 provides a larger visual obstruction than the clip alone.) When jaws 42 and 44 are compressed, bases 32 and 34 of clip 10 pivot toward one another about base end 26 and against the force of coil spring 20. That movement opens free ends 22 and 24 of blades 12 and 14. The neurosurgeon then positions opened free ends 22.and 24 of blades 12 and 14 around the vessel to be clipped. When jaws 42 and 44 are subsequently released, bases 32 and 34 of clip 10 pivot away from one another about base end 26 under the force of coil spring 20. That movement closes free ends 22 and 24 of blades 12 and 14 and clips the aneurysm in the vessel.
Distinguish an aneurysm clip from a "clamp." Clamps use malleable materials which close like a staple, lack the flexibility of a spring component, and cannot be removed. Consequently, clamps do not allow precise tailoring of the closing forces to (1) prevent dislocation, yet (2) prevent necrosis of the tissues due to overly high pressure. The clamping force is determined by how tightly the clamp is closed, not by a pre-calibrated spring force. In addition, clamps cannot form the complex shapes into which clips must be manufactured. The clip must be applied, through a very small opening, often deep inside the brain.
When operating on a deep-seated cerebral aneurysm, the neurosurgeon's visual control of the clip application is restricted by both the clip and the clip applicator. That problem has been identified, for example, in the article by A. Perneczky, "Use of a New Aneurysm Clip with an Inverted-Spring Mechanism to Facilitate Visual Control During Clip Application," J. Neurosurg 82: 898-899 (1995). Obstruction dimensions for an aneurysm clip and applicator are typically 9 mm by 5 mm. The 9 mm dimension represents the width of the clip coil (about 7 mm) plus the approximately 1 mm applicator head on either side enveloping the clip coil (see FIG. 2). These dimensions are large when compared to cerebral arteries as small as 1 mm in diameter.
One recent development (the Perneczky clip) inverts or reverses the clip action. The applicator grips the inside of the clip and does not envelop the clip. To open the clip, the applicator is opened; the applicator is closed to close the clip. This eliminates the applicator as a source of obstruction. Because the 1 mm obstruction by the applicator on either side of the clip is eliminated, the obstruction with this clip is typically reduced to the order of 7.times.5 mm.
A number of different materials are used to manufacture cerebral aneurysm clips. Most conventional aneurysm clips are limited, however, to metals and metal alloys (such as stainless steel and chrome-cobalt alloy steel) because the clips incorporate coil springs and metals and their alloys provide the necessary spring force to clip tissue. Unfortunately, most metals and metal alloys interfere with important diagnostic techniques such as magnetic resonance imaging (MRI or NMR), MRA, and CT-Scanning due to image degradation (haloing, starbursts, and "Black-Hole" obscuring) caused by the magnetic characteristic and high density of the materials. An exception is titanium, which has a very low magnetic susceptibility and density; therefore, it does not interfere with MRI, MRA, or CT-scan procedures.
Furthermore, the significant magnetic susceptibility of most metals and metal alloys presents the danger that clips made of these materials will move, rotate, or become hot in the intense electro-magnetic fields created. Aneurysm clips made of non-metallic materials including plastic, ceramic, or composites--and the exceptional metal titanium--present advantages, such as minimal interference with MRI, MRA, and CT-scan diagnostic procedures. The problem of metallic materials of construction has been discussed in U.S. Pat. No. 4,943,298 issued to Fujita et al.
The cerebral aneurysm clip of the '298 patent has blades made of synthetic resins or ceramics. The synthetic material can include, for example, fluorine or methacrylic resins or thermoplastics such as polyethylene or polypropylene. Table 1 of the '298 patent summarizes applicable ceramic materials. The advantage of such materials is disclosed as the ability to make X-ray and MRI examinations without interference from the materials. The materials also provide an advantage in that they are chemically stable and harmless to a living body, as well as being corrosion resistant and durable.
The first embodiment of the '298 patent is an otherwise standard clip improved by using plastic or ceramic material of construction. This embodiment is illustrated in FIGS. 1-5 of that patent. The second embodiment is illustrated in FIGS. 6-10 of the '298 patent. The second embodiment is a hinged clip, neither closed nor open unless biased, having blades 14a, 14b or 25a, 25b which pivot about a single point. The clip has an elastic spring member which is either compressed (see FIGS. 6 and 10) or stretched (see FIGS. 7-9) to apply a closing force on the blades. The spring member can be a sleeve shown as element 27 in FIG. 9.
The '298 patent does not disclose any way to prevent the elastic spring member from slipping on the clip. Moreover, the clipping force is not developed, in the clip of the '298 patent, by any cantilever action of the blades. Rather, the clipping force is developed by elastic springs which are made of rubber or other elastomers. Finally, the '298 patent does not disclose any type of applicator. It appears, however, that the applicator must envelop the clip and impair visibility.
Some conventional clip designs require that holes be drilled, components be welded or riveted, or recesses be formed. Machining processes are often required. Such manufacturing procedures introduce microcracks, voids, and crevices into the clip. Sharp corners of recesses and microcracks yield a clip undesirable for use as a cerebral implant. Thus, drilling, welding, riveting, and machining steps should be avoided in the processes of manufacturing an aneurysm clip; otherwise, the clip produced cannot satisfy the criteria required for a desirable clip.
The aneurysm clip disclosed by Lerch in European Patent Application No. 94108657.1 is an example of a titanium clip which requires problematic machining steps during manufacture. The clip is made from two rod halves, each half having a free end, a bump, a curved area, and a foot. The free ends of the clip halves form the clip jaws. The two rods are held together by a crimp (which has an edge or shoulder) on the feet. A ring rests on the curved area of the halves before the clip is applied. Before application, the ring jaws are spread apart. A hole is drilled through the ring and a rod is inserted in the hole so that it protrudes on either side of the ring. The ring is illustrated in FIG. 3 of the application.
The principle problem with the titanium clip disclosed by Lerch is the requirement that a crimp be provided. The crimp is objectionable, first, because it is an additional component that increases the cost of the clip and must be designed and formed with precision. Titanium and its alloys are notch sensitive; therefore, they are difficult to deform without cracking. Cracks are likely to occur when the crimp of the clip is formed. In addition, the crimp has an edge or shoulder that renders the clip undesirable for use as a cerebral implant. If a more malleable metal than titanium is used to form the crimp, the advantages of titanium would be lost and the risk of other problems (such as galvanic corrosion) arises. A crimping operation is difficult to implement with other, non-metallic materials of construction such as plastics and ceramics.
Similarly, the drilling operation on the ring may introduce microcracks, voids, and crevices into the ring. The protruding rod on either side of the ring yields undesirable extensions on a clip for use as a cerebral implant. Finally, like the crimp, the protruding rod of the ring is objectionable because it is an additional component that increases the cost of the clip and must be designed and formed with precision.
The applicator used to apply the clip has a pistol-like handle, a tube moved by the handle, and a fixed locator rod inside the tube (see FIG. 1 of the application). The locator rod has a seat with jaws on its end. With the tube pulled away from the clip, the jaws of the seat on the locator rod are positioned over the edges of the crimp on the clip (see FIG. 5 of the application). The user then slides the tube over the locator rod and forces the jaws of the seat on the locator rod around the edges of the crimp so that the locator rod holds the crimp of the clip (see FIG. 6 of the application). The user continues to slide the tube over the locator rod until an uptake slot on the end of the tube engages the protruding rod on either side of the ring. Using a wheel, the tube is rotated so that the uptake slot "catches" the protruding rod. Finally, the user slides the tube until the clip is within the tube and the ring is positioned over the bumps on the clip halves. This action forces the jaws of the clip together. (See FIG. 7 of the application.)
The applicator disclosed by Lerch surrounds the clip. Therefore, the applicator is larger than the clip and restricts the view of the neurosurgeon. During application, the neurosurgeon must accomplish the additional procedural step of rotating the tube so that the uptake slot of the applicator tube catches the protruding rod of the clip ring. This introduces another inconvenient and time-consuming procedural step (requiring the use of two hands), however, and is undesirable because the uptake slot may fail to catch the clip ring unless the step is performed correctly.
To overcome the shortcomings of existing aneurysm clips, a new cantilever aneurysm clip system is provided that reduces visual obstruction. An object of the present invention is to provide an improved aneurysm clip incorporating a cantilever spring force. A further object is a design that does not require coil springs and that can be easily manufactured from almost any material, including titanium, ceramic, plastic, or composites. It is still another object of the present invention to provide an improved applicator that is positioned next to, rather than around, the aneurysm clip, to further improve visual control. Another object of the present invention is to achieve an adequate closing force using less spring material than is required by conventional coil clips; therefore, the weight of the clip is reduced.